When a joint, such as the hip or shoulder, becomes impaired due to arthritis, disease or trauma, it is sometimes necessary to replace all or part of the joint with a prosthesis to restore function. For instance, hip replacement, where a prosthesis is provided to replace the femoral head and in some cases all or part of the acetabulum, has become a common procedure to treat femoral head fractures and arthritis in elderly patients. As a result of anatomical constraints and challenges in the shoulder, shoulder implants have historically been much less successful and less common than hip replacements. Recently, however, shoulder arthroplasty has emerged as an accepted treatment for severe arthritis and humeral head fractures.
As a consequence of the increasing acceptance of shoulder prostheses, many different devices have been developed to address various problems that have arisen and to offer additional benefits and features. In the simplest form, a shoulder prosthesis is formed as a single piece with a head to articulate with the glenoid cavity, and a stem to extend down the medullary canal of the humerus and support the head. While simple to construct, unitary implants do not offer any adjustability to accommodate the natural variations in size and geometry that occur among joints of different patients. To accommodate these variations, a large stock of devices must be manufactured and maintained to insure that an adequate match can be achieved during an operation. Stocking the large number of devices is a significant expense with one-piece designs, and in some cases a surgeon may not be provided with sufficient flexibility to achieve an ideal fit to the patient.
To avoid the expense of maintaining a large stock of single-piece prosthetics and to provide increased flexibility to surgeons, many shoulder implant makers have gone to a modular design that is assembled during the operation from two or three pieces. These pieces include a head to articulate with the glenoid and a stem structure on which the head is mounted and secured to the bone. In some cases, the stem includes a separate body portion disposed between the head and an intermedullary portion of the stem that extends down the medullary canal. By utilizing a modular design, a wide variety of devices can be assembled from only a few pieces, thus providing increased flexibility to accommodate anatomical variation and eliminating much of the cost associated with maintaining a large selection of one-piece devices.
Existing modular shoulder designs most commonly rely on a taper lock mechanism to secure the head to the rest of the implant. In at least some devices the portion of the taper lock on the head is offset to compensate for anatomical posterior offset of the humeral head. For instance, the taper lock portion on the head may be offset by 2-4 millimeters. By rotating the head, any offset between plus and minus the 2-4 millimeters can be achieved. Unfortunately, rotating an offset head can introduce a medial/lateral and/or superior/inferior offset at the same time the anterior/posterior positioning is adjusted. Furthermore, the offset between the center of the taper lock and the geometrical center of the head creates a torque which tends to rotate the head relative to the remainder of the implant, thereby increasing the chance of loosening of the head. As the offset increases, the resultant torque increases as well, making this a greater problem for larger offsets. Although such problems are incumbent in existing offset head designs, a posterior offset is generally desirable to better match the natural anatomy.
In addition to the specific drawbacks associated with various existing implant designs, there are a number of general problems inherent in shoulder replacements. In particular, it is generally difficult to establish the proper position and orientation for the implant in the humerus. One of the more important variables is the rotational position, or retroversion, of the head on the humerus. Anatomically, the average retroversion between a plane defmed by the perimeter of the anatomical head and the axis of the flexed forearm is approximately 30-degrees. Unfortunately, with existing implants and techniques for their installation, it has been very difficult to reliably reproduce desired retroversion. Establishing correct retroversion is important because incorrect retroversion can lead to problems with subsequent dislocation.
In addition to the retroversion of the implant, it is necessary to establish the correct height of the implant on the humeral shaft. With existing designs, the surgeon slips the stem into the medullary canal and makes an educated guess at the proper height. Excess height may create too much tension in the deltoid, while inserting the implant too far down the humerus can result in deltoid lag. Similarly, the offset of the face of the head relative to the stem must be established correctly or excess or insufficient tension in the rotator cuff may be created. Unfortunately, with existing designs there is no way to evaluate implant height or head offset prior to final installation, after which correction is difficult.